Slitherine

Where can I see the XP of units.
How to upgrade them to a higher tech level - what are the requisites to be able to?
How to win the sub war as GB?

If you hover your mouse over the strength of the unit in the unit panel then you can read the XP. Max XP is 100. Every 25 XP you get a new XP level which will increase some of the unit attributes (like ground attack or antitank). You can see the XP levels as dots under the unit symbol on the map.

You buy labs using the Research button. When your get new tech levels then your units can be upgraded using the Upgrade button. That will highlight all eligible units. On the units you will see numbers current number of techs / max number of techs with current tech levels. If the max number is higher than the current number then the unit can be upgraded. If you look into the unit panel when hovering over a unit while doing upgrade you can see color changes on certain unit attributes. That means these values will be increased if you upgrade.

In GS v3.20 the unit XP will be shown directly behind the unit strength in the unit panel like this:
10 / 23.

That means strength 10 and 23 XP. This will make it even easier to notice the current number of XP.